The last thirty years has seen rather widespread installation of strong-motion accelerographs in tall buildings, oil refineries, power plants, dams, and other critical structures. Many records of earthquake ground motion from these and other installed instruments became available. Numerous such records have been analyzed and compared with the actual damage caused by the earthquakes --a procedure known as post-event analysis.
Several rules of interpretation have been established and applied to such post-event analysis. For example, a study funded by electric utility companies and conducted by the Electric Power Research Institute (EPRI) compared seismic data and damage effects for over 260 earthquakes. One of the findings was that ground accelerations at frequencies below 10 Hertz had significantly greater damage potential than at higher frequencies. It was further found that the duration of an event (earthquake) was an essential factor in assessing damage potential. EPRI then concluded that a computed quantity called Cumulative Absolute Velocity (CAV) provided the most significant determinant of earthquake damage potential. CAV is determined by summing the accelerations in both directions in the frequency range 0-10 Hz and multiplying them by their duration in seconds, which is equivalent to the area under the acceleration vs. time curve. CAV is expressed in g-seconds, where g is the acceleration of gravity. It may be noted that CAV is so called because it has the dimensions of length divided by time, or velocity. Typical values of CAV may range from around 0.1 to 5.
For certain applications it is convenient to provide scales of earthquake damage potential based on CAV but over different frequency ranges and having generally larger numerical values, to avoid confusion with the existing Richter and Mercalli scales. These may be called Engdahl scales, or ES. An Engdahl scale is defined as the CAV multiplied by 100, where the acceleration data have been filtered through a specified low-pass filter before being integrated against time. At present there are four Engdahl scales, which differ only in the filtered frequency band. They are:
______________________________________ ES (L) Low band 0-4 Hz ES (M) Medium band 0-12.7 Hz ES (H) High Band 0-40.4 Hz ES (V) Very High Band 0-128 Hz. ______________________________________
The L and M scales relate primarily to structural damage from earthquake ground motion. The H and V scales relate mainly to damage caused by blasts, where forces are applied very suddenly.
A rough comparison may be made between the Engdahl scales and the Richter and the Modified Mercalli scales. The Richter scale is a geophysical scale that expresses logarithmically the total amount of energy released in the whole region of an earthquake. Ground motion at any given point, however, varies with the nature of the underlying strata and the location and depth of the focus (epicenter) of the quake. The Modified Mercalli (MM) scale is a qualitative expression of the structural damage and social effects of an earthquake at a given location. A portion of the MM scale is summarized below:
MMI IV: Felt indoors by many, awakened a few. Some rattling of windows or creaking of walls. PA1 MMI V: Felt by most; slight excitement, frightened a few. Some broken dishes. PA1 MMI VI: Felt by all; frightened many; excitement general; some alarmed. Damage slight; some broken windows. PA1 MMI VII: General alarm; damage moderate in many ordinary buildings, considerable damage in poorly built buildings and old walls; many fallen chimneys. PA1 MMI VIII: Alarm approaches panic. Considerable damage in some ordinary buildings, many fallen walls, chimneys, towers; some tumbled wooden houses. PA1 MMI IX: Panic general. Cracked ground conspicuous. Damage considerable in some buildings specially designed for earthquakes. Damaged reservoirs and underground pipes. PA1 (a) Integrate the accelerometer data with respect to time; PA1 (b) Discard data below a preselected acceleration level; PA1 (c) Filter the accelerometer data to pass frequencies below a preselected value, e.g. to pass 0-4 Hz; PA1 (d) Display the integrated data numerically and continuously; PA1 (e) Periodically re-zero the integrator to prevent the accumulation of irrelevant data, as from the instrument becoming slightly tilted; PA1 (f) Provide an alarm signal and relay closure when the integrated data (cumulative velocity) exceeds a predetermined value; and optionally, a electrical output signal capable of closing a suitable gas valve or initiating shutdown of critical machinery. PA1 (g) Provide storage of data for future access.
Although the methods of gathering and interpreting data differ, an approximate equivalency chart is shown below:
TABLE I ______________________________________ Engdahl Modified Richter Scale Mercalli Magnitude ______________________________________ 5-11 IV 4.0-4.5 11-23 V 4.5-5.0 23-48 VI 5.0-5.5 48-98 VII 5.5-6.0 98-200 VIII 6.0-6.5 200-410 IX 6.5-7.0 ______________________________________
It will be apparent that an instrument that determines CAV or Engdahl intensities immediately at the time of an earthquake may: (1) furnish data in a single number for correlation with structural damage, and (2) operate to energize alarm devices and to automatically shut down critical apparatus or machinery, as, closing gas valves or shutting down pumps.